In the last few decades, significant efforts have been devoted to improving the activity and durability of Pt based nanomaterials for oxygen reduction reaction (ORR) aiming to overcome the high price and scarcity problems of Pt based catalysts for fuel cell application in stationary and automobile sectors. Since the discovery of the 90-fold activity improvements of nano-segregated Pt3Ni(111) surface in comparison to commercially available platinum on carbon (Pt/C) catalyst, a variety of nanocrystals with different shapes and significant improved activities have been developed, in most cases, using solution-phase synthesis methods. In contrast to Pt/C catalyst prepared by impregnation method, these nanocrystals made by solution-phase synthesis usually have well controlled shape and composition and monodispersed particle size; thus, their surfaces could be fine-tuned to give much higher catalytic activities. However, usually these highly active nanocrystals can only be made in milligram scale because great attention has to be paid in the synthesis and post-treatment processes in order to keep the desired morphologies, particle sizes, and surface composition profiles. This is barely enough for physical characterizations and electrocatalytic activity testing on rotating disk electrode (RDE), their performances in real fuel cell membrane electrode assembly (MEA) have been rarely investigated which greatly hinders the commercialization of fuel cell technology. Gram scale of high quality nanocrystal catalyst should be made available for MEA testing, in order to bridge the gap between fundamental researches and real device investigations.
Multi-layered Pt-skin nanoparticle catalyst have been synthesized by solution-phase synthesis with a series of post-treatment processes. Solution-phase synthesis provides the advantage of shape, size and composition control. U.S. Pat. No. 9,246,177 describes one process for monodispersed PtNi bi-metallic nanoparticles, such as with a size of around 5 nanometers (nm), using a hot-injection synthesis method in organic solvent. Prior techniques for synthesis have utilized a hot injection method that requires a multi-step process of 1) injection facilitated synthesis, 2) loading on carbon, 3) acid leaching, and 4) annealing. In step 1, a mixture of nickel acetate tetrahydrate, 1,2-tetradecanediol, oleic acid, oleylamine, and either diphenyl ether or dibenzyl either is heated to 200° C. To this heated mixture, a pre-heated (˜80° C.) Pt solution is injected. The Pt solution comprises platinum (II) acetylacetonate in 1, 2-dichlorobenzene. The combined solution is then held at 200° C. for an hour. After separation from synthesis solvents, in a second step, the PtNi nanoparticles are loaded onto high surface area carbon by evaporating off the solvents from the first step. Next, in a third step, the resultant loaded PtNi/C material is acid leached by sonicating directly in the acidic solution and then centrifuged to separate the acid. In a fourth step, the acid leached PtNi/C undergoes annealing in hydrogen atmosphere to smoothen the surface and form a Pt-skin surface structure. With all these sophisticated synthesis and post-treatment procedures, a nanoparticle catalyst with multi-layered Pt-skin structure similar to that on bulk thin film surface was prepared which shows over 6-fold activity enhancement and significant durability improvement compared with commercial Pt/C catalyst.
However, this process exhibits several drawbacks and flaws that hinder scaling of the process. Further, the resultant product is a typical solid nanoparticle with size less than 5 nanometers. Fundamental researches reveal that bulk alloy surfaces usually show significant performance and durability benefit over the surfaces of small nanoparticle catalysts which have lots of low-coordinated Pt atoms. However, bulk alloy or bigger nanoparticles usually have very limited surface areas which limit their real application. In this regards, a cage structure which have larger general particle size (thus less low-coordinated Pt atoms on surface) but still high surface area (because of the hollow structure) is promising structure for both high performance (activity and durability) and high surface area.